1. Field of the Present Invention
The invention relates generally to the field of precision motion control, and, in particular, to the ability to measure workpiece form error and surface finish.
2. Background
In general, the use of instruments for profiling the surface of a workpiece, often referred to as “profilometry,” represents a crucial method or apparatus for extracting precision measurement of surface parameters. In particular, profilometry is used to measure a workpiece's profile or surface finish. Due to the accuracies of these instruments, parts are measured to assess their compliance with specification requirements and the manufacturing process is adjusted accordingly to reduce the scrap of parts. In other words, surface profilometry is a tool to maintain quality of parts produced within the manufacturing process.
On common type of profiling instrument is a contact based stylus sensor. For example, in U.S. Pat. No. 6,327,788, a stylus is traversed about a cylindrical surface and the deflection of the stylus is measured. In this arrangement, the deflection is caused by the spatial interaction between the stylus tip and the surface of the workpiece. For example, a change in local slope or surface roughness will change the deflection of the stylus. Unfortunately, although this method provides accurate measurements, its scanning speed is limited due to the low resonating behavior of the stylus or the high inertia of the scanning apparatus. The slow scanning speed, in turn, results in slower production cycles, potentially increased scrap and increased cost for each item being produced. Furthermore, designing an instrument similar to the stylus, but with a higher resonant frequency, may result in a less sensitive device with the inherent inability to measure small features. In addition, the contact force between the stylus and the surface varies because of the spring force that is needed to keep the stylus in contact with the surface.
Various attempts have been made to speed up the surface inspection process. For example, in U.S. Pat. No. 5,189,806, a scanning head is translated, relative to a workpiece, along an axis while a probe or stylus mounted on the scanning head is oscillated from side to side. This permits an entire area (“A”) to be scanned on one pass of the scanning head. As a result, the entire surface area of a workpiece could be scanned with fewer time-consuming repositioning movement of the scanning head, thereby reducing the overall amount of time required to process a workpiece. Unfortunately, the speed of the scanning process itself was still limited for reasons similar to that of the approach of U.S. Pat. No. 6,327,788. U.S. Pat. No. 5,189,806 requires the use of electric motors and air bearings, devices that once again demonstrate low resonating behavior and high inertia.
One significant factor inhibiting the scanning speed in known approaches is the failure of prior approaches to maintain a constant force between the probe and the surface of the workpiece. For example, U.S. Pat. No. 5,189,806 generally relies on a constant torque at the drive motor to derive a constant normal load in a direction tangential to the axis of rotation of the motor upon which the probe is mounted. Although that patent apparently recognizes the utility of including a strain gage or other force monitoring means for detecting deformations of the probe, it does not disclose any means for using such force measurements to control the probe itself.
Not only does this lack of a feedback control loop limit the scanning speed in and of itself, but in addition, running the motor at constant force inherently limits the stiffness of the system. In this case, the stiffness of the probe is predominantly due to contact between the probe tip and solid surface. This will clearly vary from material to material as well as with surface geometry. Thus, a need exists for a system in which the dynamics are obtained from the probe itself, therefore enabling scanning at arbitrary forces and being little changed when measuring soft specimens.
In addition, circular features (i.e. inner diameters and outer diameters) are of significant concern within the field of profilometry. For example, optical fiber connectors often consist of a cylindrical ferrule with a small hole into which the optical fiber is bonded. Because the bore diameter of the ferrule is typically very small (typically only a small fraction of a millimeter), prior art scanning instruments cannot enter the holes to measure the internal surface profile. Additional examples of circular features requiring surface profiling may include cylinder bores, riveted holes for aerospace, hydraulic spool valve housings, injectors, fiber optic coupling and bearings. Thus, a particular need exists for a method and apparatus for profiling small circular surfaces.